1. Technical Field of the Invention
The present invention relates generally to electronic amplifiers, and in particular, to a reduced power consumption amplifier that can provide power from multiple voltage sources.
2. DESCRIPTION OF THE RELATED ART
Various types of electronic amplifiers are known. FIG. 1 shows an example of a conventional Class AB amplifier 5. Class AB amplifiers use an arrangement of pnp and npn transistors Q1, Q2 known as complementary pair 6. The pair can be biased by voltage generators 7-8. In response to an input voltage, the Class AB amplifier 5 produces an output voltage V.sub.OUT that follows the input voltage.
Although Class AB amplifiers provide an efficient means for transferring power to loads, a simple Class AB amplifier can provide power from only one voltage supply. In many applications, this causes inefficiency because voltage supplies are optimized to perform over specified ranges. Consequently, it is known to be more efficient to supply power from multiple sources. For example, in the field of ultrasound imaging, it is generally more efficient to deliver transmit power to an ultrasound imager from several voltage sources, where each source optimally performs over a predetermined voltage range.
FIG. 2 illustrates a prior art transmit voltage selector circuit 11 for accomplishing power supply selection in a ultrasound imaging system 10. The transmit selector 11 includes a microcontroller 12 and a plurality of software controlled switches 14a-d, which are coupled to a plurality of voltage supplies 16a-d. The voltage selector 11 supplies power to an ultrasound imager 18 by discretely selecting the supplies best suited for the load requirements. Based on the power requirements of the imager 18, the microcontroller 12 selects the one voltage supply from the group 16a-d that can supply the load voltage most efficiently. One drawback of this arrangement is that the supply voltage must pass through a software-controlled switch, which results in a discontinuous, switched output that can cause noise in the imager 18. Another problem is that the frequency response of the software-based selector 11 may not be fast enough to keep up with changes in the power requirements of the imager 18. The selector 11 is also relatively complex and costly. For instance, to select the appropriate supply, the microcontroller 12 must be programmed according to the amplifier transfer function of the ultrasound imager 18. This increases the complexity of the software in the microcontroller 12. Moreover, the microcontroller l 2 requires control inputs for the selection function. These inputs must be supplied by external logic and circuitry, which additionally increases the cost and complexity of the system.
Thus, there is a need for a device that simplifies and improves the delivery of transmit power in an ultrasound imager.